Cartilage is an avascularized tissue and has been studied as a potential candidate containing anti-angiogenic factors. It is also a tissue which is relatively resistant to tumor development. The tumor associated with cartilage, chondrosarcoma, is the least vascularized of solid tumors. Angiogenesis is one of the important factors in the development of a tumor. Discrete solid tumoral masses appear if the tumor cells can provoke the adjacent vascular network to expand to supply their nutritional needs. Therefore, the factors involved in the stimulation of angiogenesis have been studied for their role in the development of tumor and anti-angiogenic factors as well as drugs having an angiogenic inhibitory activity have been also investigated as tools for controlling the growth or for effecting regression of tumors.
It has been discovered that scapular cartilage in calves contains a substance that inhibits the vascularization of solid tumors (Langer et al., 1976). Because of its encouraging potential as anti-tumor agent, sources of greater supply of cartilage have been looked for.
Sharks are animals being a potential source of this kind of angiogenesis inhibitor because their endoskeleton is composed entirely of cartilage (6% of their body weight versus 0.6% in calves). Sharks have also as an interesting characteristic a low propensity to developing tumors. Many hypotheses have been elaborated to explain this low probability of developing tumors in sharks. Marchalonis et al. (1990) have shown IgM antibodies able to readily attack any aggressing agent. McKinney et al. (1990) have shown that sharks have macrophages capable of differentiating normal cells from neoplastic cells and of destroying the latter. Rosen and Woodhead (1980) have postulated that the rarity of tumors in elasmobranchs (a group to which pertain sharks and rays) might be due to the high ionic strength of their tissues, which is equivalent to a high body temperature. In these conditions, these authors believe that the immune system exerts a close to 100% immunological surveillance. Moore et al. (1993) have discovered that sharks produce an aminosterol having antibacterial and antiprotozoal properties. Finally, Lee and Langer (1983) and Folkman and Klagsbrun (1987) have shown that sharks produce a substance which inhibits neovascularization. Lee and Langer (op.cit.) have isolated this substance by extracting it from shark cartilage in denaturing conditions (guanidine extraction). This process of extraction is however very long (41 days) and might generate extracts having denatured factors. While the active substance isolated from calves has a molecular weight of about 16 kilodaltons (kd), the same group of researchers have not given a precise molecular weight to the one retrieved in sharks. This substance is only defined has having a molecular weight higher than 3500 daltons. Oikawa et al. (1990) have applied the same method of extraction as the one described by Lee and Langer, but of a much shorter duration (2 days instead of 41 days). The substance isolated from shark cartilage by Oikawa et al. has a molecular weight ranging from 1000 to 10000 daltons. Schinitsky (U.S. Pat. No. 4,473,551) has described a water extract of crude powdered shark cartilage which fraction of more than 100,000 Daltons has an anti-inflammatory activity alone or in combination with glucosamine. No suggestion of a component of this extract having an anti-angiogenic or anti-tumor activity is made in this patent. Kuetner et al. (U.S. Pat. No. 4,746,729) have isolated a polymorphonuclear neutrophil (PMN) elastase inhibitor from bovine cartilage. This inhibitor has been obtained from an aqueous extract of cartilage from which molecules of a molecular weight of more than 50,000 Daltons have been retained. Fractionation on Sephacryl S-200 has given numerous fractions from which those of 10-40 kD have been pooled after they have demonstrated an anti-elastase activity. The active component has an isoelectric point of 9.5 and might have a molecular weight of about 15,000 Daltons. Kuetner et al. (U.S. Pat. No. 4,042,457) have also shown that bovine cartilage has a component of a molecular weight of less than 50,000 Daltons which has a cell proliferation inhibitory activity without any activity on endothelial cell growth. Spilburg et al. (U.S. Pat. No. 4,243,582) have isolated two glycoproteins of molecular weight of 65 KD and of pI 3.8 from bovine cartilage (guanidine-extraction) which show anti-trypsin activity and an endothelial cell growth inhibitory activity.
Calf and shark cartilage contain many biological activities such as lysozyme activity, cell growth-promoting activity, inhibitory activity against types I and IV collagenase, elastase, and against proteases like trypsin, chymotrypsin and plasmin.
Methods of obtaining a shark cartilage extracts and fractions are already known. Some of them produce a powdered crude cartilage without any extraction (U.S. Pat. No. 5,075,112), others use denaturing agents like guanidine (U.S. Pat. No. 4,243,582), or enzymatic reaction to get rid of any muscular, nervous or vascular structures surrounding the cartilage and organic solvents to eliminate fats (U.S. Pat. Nos. 3,478,146, 4,350,682 and 4,656,137) and to obtain an extract of cartilage, while others simply produce aqueous extracts (in water (U.S. Pat. No. 4,473,551) or salt solutions (U.S. Pat. No. 4,746,729)) of cartilage by eliminating the unsolubilized material. Among the latter, specific fractions of specific molecular weights have been particularly retained for further study and purification (see discussion above).
Summing up, shark cartilage is known as containing anti-angiogenic component(s) generally tested in rabbit corneal pocket assay or in chick chorioallantoic membrane (CAM) assay. Up to date, whole powdered cartilage has been tested directly on tumors in vivo, on human melanoma xenograft implanted in nude mice (U.S. Pat. No. 5,075,112), as well as tested in CAM for its anti-angiogenic effect. However, no evidence has been brought to a direct effect of cartilage extract on tumor cells.
Angiogenesis is not only involved in cancer development. Many diseases or conditions affecting different physiological systems (indicated in parentheses) are angiogenesis-dependent among which the following examples: arthritis and atherosclerotic plaques (bone and ligaments), diabetic retinopathy, neovascular glaucoma, trachoma and corneal graft neovascularization (eye), psoriasis, scleroderma, hemangioma and hypertrophic scarring (skin), vascular adhesions and angiofibroma (blood system). Therefore, any new anti-angiogenic factor could find a use in the treatment of these diseases as well as in cancer therapy.